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21 Wireless Local Area Networks
a local area. The three physical
layers are an IR baseband PHY, an FHSS radio
in the 2.4 GHz band, and a DSSS radio in the 2.4 GHz. All three physical layers
support both 1 and 2 Mbps operations.
WLANs support asynchronous data transfers that refer to the traffi c that is rel-
atively insensitive to time delays such as electronic mail and fi le transfers. Optionally
WLANs can also support the traffi c, which is bounded by the specifi ed time delay, to
achieve an acceptable quality of service (QoS), such as packetized voice and video.
21.5.1 IEEE 802.11 Architecture
The architecture of the IEEE 802.11 WLAN is designed to support a network
where most decision making is distributed to mobile stations. This type of archi-
tecture has several advantages. It is tolerant of faults in all of the WLAN equipment
and eliminates possible bottlenecks a centralized architecture would introduce.
The architecture is fl exible and can easily support both small, transient networks
and large, semipermanent or permanent networks.
In addition, the architecture
and protocols offer signifi cant power saving and prolong the battery life of mobile
equipment without losing network connectivity.
Two network architectures are defi ned in the IEEE 802.11 standard:
Infrastructure network:
An infrastructure network is the network archi-
tecture for providing communication between wireless clients and wired
network resources. The transition of data from the wireless to wired medium
occurs via an AP. An AP and its associated wireless clients defi ne
the coverage
area. Together all the devices form a
basic service set
(see Figure 21.5).
Point-to-point (ad hoc) network:
An ad hoc network is the architecture
that is used to support mutual communication between wireless clients.
Typically, an ad hoc network is created spontaneously and does not support
access to wired networks. An ad hoc network does not require an AP.
IEEE 802.11 supports three basic topologies for WLANs: the independent
basic service set (IBSS),
the basic service set, and the extended service set (ESS).
The MAC layer supports implementations of IBSS, basic service set, and ESS
confi gurations.
The IBSS confi guration is referred to as an independent confi guration or an
ad hoc network. An IBSS confi guration is analogous to a peer-to-peer offi ce
network in which no single node is required to act as a server. IBSS WLANs
include a number of nodes or wireless stations
that communicate directly
with one another on an ad hoc, peer-to-peer basis. Generally, IBSS imple-
mentations cover a limited area and are not connected to any large network.
An IBSS is typically a short-lived network, with a small number of stations,
that is created for a particular purpose.
The basic service set confi guration relies on an AP that
acts as the logical server
for a single WLAN cell or channel. Communications between station 1 and
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station 4 actually fl ow from station 1 to AP1 and then from AP1 to AP2 and
then from AP2 to AP4 and fi nally AP4 to station 4 (refer to Figure 21.4). An
AP performs a bridging function and connects multiple WLAN cells or chan-
nels, and connects WLAN cells to a wired enterprise LAN.
The ESS confi guration consists of multiple basic service set cells that can
be linked by either wired or wireless backbones called a distributed system.
IEEE 802.11 supports ESS confi gurations in which multiple cells use the
same channel, and confi gurations in which multiple cells use different chan-
nels to boost aggregate throughput. To network the equipment outside of
the ESS, the ESS and all of its mobile stations
appear to be a single MAC-
layer network where all stations are physically stationary. Thus, the ESS
hides the mobility of the mobile stations from everything outside the ESS
(see Figure 21.5).
